Nuclear Magnetic Resonance Spectroscopy With Answers Page 19
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CHAPTER 16
OH
OH
OH
O
16.60.
16.61.
16.62.
OMe
O
16.63.
16.64.
16.65. N,N-dimethylformamide (DMF) has several resonance structures:
O
O
O
CH
CH
CH
3
3
3
H
N
H
N
H
N
CH
CH
CH
3
3
3
Consider the third resonance structure shown above, in which the C-N bond is a double
bond. This indicates that this bond is expected to have some double bond character. As
such, there is an energy barrier associated with rotation about this bond, such that rotation
of this bond occurs at a rate that it slower than the timescale of the NMR spectrometer.
At high temperature, more molecules will have the requisite energy to undergo free
rotation about the C-N bond, so the process can occur on a time scale that is faster than
the timescale of the NMR spectrometer. For this reason, the signals are expected to
collapse into one signal at high temperature.
16.66. In a concentrated solution of phenol, the OH groups are engaged in extensive,
intermolecular hydrogen-bonding interactions. These interactions cause the average
distance to increase between the O and H of each OH group. This effectively deshields
the protons of the hydroxyl groups. These protons therefore show up downfield. In a
dilute solution, there are fewer hydrogen bonding interactions, and the effect described
above is not observed.
16.67. The methyl group on the right side is located in the shielding region of the π bond,
so the signal for this proton is moved upfield to 0.8 ppm.
16.68. Bromine is significantly larger than chlorine, and the electron density of a
bromine atom partially surrounds any carbon atom attached directly to the bromine,
thereby shielding it. In CBr
, the carbon atom in the center of the compound is
4
significantly shielded because it is positioned within the electron clouds of the four
bromine atoms. In fact, it is so strongly shielded that it produces a signal even higher
upfield than TMS.
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